Gave up trying to reset env var - set to "*" after test

And after doctest. it stayed at "*" after the last test anyway, letting
the doctest fail.

doctests are not recommended for side effectful tests anyway https://hexdocs.pm/ex_unit/main/ExUnit.DocTest.html

lib/basic_signup_whitelist.ex

@@ -18,8 +18,12 @@ defmodule BasicSignupWhitelist do
 
   ## Examples
 
+      iex> System.delete_env("SIGNUP_ALLOWED_EMAILS")
       iex> BasicSignupWhitelist.mail_whitelisted("joe@example.com")
       false
+      iex> System.put_env("SIGNUP_ALLOWED_EMAILS","*")
+      iex> BasicSignupWhitelist.mail_whitelisted("joe@example.com")
+      true
 
   """
   def mail_whitelisted(email) do

test/basic_signup_whitelist_test.exs

@@ -1,33 +1,29 @@
 defmodule BasicSignupWhitelistTest do
-  use ExUnit.Case
+  use ExUnit.Case, async: false
   doctest BasicSignupWhitelist
 
   defp allow_signups_for(whitelist) do
     System.put_env("SIGNUP_ALLOWED_EMAILS", whitelist)
   end
 
-  setup do
-    # Save original env var
-    original_env = System.get_env("SIGNUP_ALLOWED_EMAILS")
+  describe "setup in describe block" do
+    setup do
+      on_exit(fn ->
+        # resetting values did not work, so 'just' set it to the effect without this module configured.
+        System.put_env("SIGNUP_ALLOWED_EMAILS", "*")
+      end)
 
-    on_exit(fn ->
-      # Restore original env var after each test
-      case original_env do
-        nil -> System.delete_env("SIGNUP_ALLOWED_EMAILS")
-        value -> System.put_env("SIGNUP_ALLOWED_EMAILS", value)
-      end
-    end)
+      :ok
+    end
 
-    :ok
-  end
+    test "When not set, not whitelisted" do
+      System.delete_env("SIGNUP_ALLOWED_EMAILS")
+      refute BasicSignupWhitelist.mail_whitelisted("joe@example.com")
+    end
 
-  test "When not set, not whitelisted" do
-    System.delete_env("SIGNUP_ALLOWED_EMAILS")
-    refute BasicSignupWhitelist.mail_whitelisted("joe@example.com")
-  end
-
-  test "When set to star, whitelisted" do
-    allow_signups_for("*")
-    assert BasicSignupWhitelist.mail_whitelisted("joe@example.com")
+    test "When set to star, whitelisted" do
+      allow_signups_for("*")
+      assert BasicSignupWhitelist.mail_whitelisted("joe@example.com")
+    end
   end
 end